premarital sex

Finer said the likelihood of Americans having sex before marriage has remained stable since the 1950s, though people now wait longer to get married and thus are sexually active as singles for extensive periods.

I'd guess that the pill and Griswold v. Connecticut changed a lot, but I've heard anecdotes of swinging back in the late 1930s (and, albeit as "reports of reports", "petting parties" in the '20s), so it would have been cool if they could have taken this data back further than they did. (via Rebecca Blood)